Net changes in antibiotic concentrations downstream from an effluent discharge

Many studies have shown the occurrence of antibiotics and degradation products in streams; however, relatively little work has applied a functional perspective to antibiotic transport and uptake. This study examined net changes in antibiotic concentrations downstream from a wastewater treatment plant (WWTP) effluent discharge and estimated net uptake length (Snet), net uptake velocity (v(f-net)), and net areal uptake rate (Unet) of antibiotics over a 3-km stream reach at Mud Creek, northwest Arkansas, USA, during June, September, and December 2006. Ten antibiotics and one degradation product (azithromycin, ciprofloxacin, erythromycin, erythromycin-H2O, ofloxacin, sulfachloropyridazine, sulfadimethoxine, sulfamethoxazole, tetracycline, oxytetracycline, and trimethoprim) were found at least once at Mud Creek downstream from the effluent discharge. All chemicals persisted in measurable concentrations in the water column over the 3-km stream reach, and we observed significant net retention of some antibiotics and one degradation product across the sampling events. Antibiotics that were significantly retained traveled kilometer-scale distances (Snet: 1.8 to 51.5 km) with relatively low uptake velocities (v(f-net): 1.6 to 33.9 x 10(-6) m s(-1)) and rates (Unet: 0.01 to 38.4 x 10(-6) microg m(-2) s(-1)). This study illustrates that some antibiotics do not travel conservatively in streams and that uptake processes occur over the scale of kilometers, linking upstream effluent sources to downstream processing over large spatial scales.